Looking for Consequences of Shifting Seasons

Willow Warbler at sea, resting on board during autumn migration. Close to the Buiten Ratel on the Belgian part of the North Sea. (Photo: Hans Hillewaert)

In a small park at Wageningen University, biologist Arnold van Vliet points out the signs of spring that are all around–a prunus tree, with nice white flowers, a hazel bush unfolding its leaves. It's a lovely sight on a beautiful spring day.

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The only problem is that these flowers and leaves really shouldn't be here yet.

"Everything is now two to three weeks ahead of schedule," van Vliet says. "Butterflies are appearing very early–extremely early because of the very warm March we had."

But a warm March here isn't that much of an anomaly these days. Van Vliet says spring is regularly coming weeks earlier than it used to in the Netherlands. In fact, he says, with temperatures on the rise the whole climate of the country has shifted in the past 10 years to become more like southern France.

Van Vliet has been following this climatic shift for more than a decade as head of an effort here called "Nature's Calendar." The program enlists the help of more than 8,000 scientists and ordinary Dutch citizens to track changes in the seasons through what's known as phenology. That's an old-fashioned word for the study of the timing of seasonal, life-cycle events, such as the first flowering of a particular plant, or when a species of bird first lays its eggs in spring.

People who work close to nature have been tracking this kind of data for centuries. But environmental scientists in the Netherlands and elsewhere are more concerned about it than ever, because the shifting of the seasons is having real environmental effects.

"We see that the length of our growing season is already one month longer than before 1988, when the temperature started to change," van Vliet says. "We see already an enormous change in species diversity in the Netherlands–very many southern species that live in Belgium, France and even farther south, that (now) appear in the Netherlands. And the more cold-loving species are significantly decreasing. So we see that signal."

And the Dutch aren't alone. As the planet warms up, scientists are seeing a similar trend around the world. Jake Weltzin, an ecologist with the US Geological Survey in Tucson, Arizona, and coordinator of the USA National Phenology Network, says spring has been coming earlier in much of the United States.

Weltzin says maple sap started running earlier than normal this year, and species as different as butterflies and horseshoe crabs turned up earlier as well.

Many plants and animals can adapt to earlier springs, Weltzin says, but he adds that what's really important is how these shifts in timing can affect an entire landscape.

"We're starting to get a handle on that as a complex system," he says. Among other things, he says there's more potential for what he calls "mismatches" in an ecosystem.

"If you have a plant, and an animal, and the animal depends on that plant for nectar, or forage or food, and if the plants are coming early, and the animals aren't arriving at the same time, you can end up with this mismatch. So there may not be enough pollination, there may not be enough food. There may not be enough milkweed for the Monarch butterflies."

In some cases, these mismatches that are coming with the shifting seasons could even affect food crops for people.

Weltzin says what's needed to better understand these trends and changing relationships is greater cooperation and data sharing among national phenology networks.

Like the Netherlands, many European countries have such networks, and in fact there is now a European database of historical phenological data.

Kjell Bolmgren, who directs Sweden's National Phenology Network, says the data suggest that spring is coming a week earlier there than it used to. But he notes that isn't necessarily bad news for all of Sweden's plants and animals.

"My prediction would be that most organisms in Sweden will benefit from the improved growing conditions," Bolmgren says, "simply because the difficult part in Sweden is the winter. So once that gets shorter, it's going to be easier for most plants."

Bolmgren says it also means a longer growing season in Sweden, but he cautions that the Swedish summer might get so long that drought becomes a problem.

There's also no guarantee that important parts of the country's ecosystems won't get out of whack.

Arnold Van Vliet at Wageningen University says he's already seeing winners and losers in the Netherlands.

Plants and insects seem to be adapting fairly quickly to the earlier Dutch springs, he says, but migratory birds do not seem to be getting the clue to come back sooner. And he says that could mean trouble for some ecological relationships.

Still–what's the big worry about all this?

"Yeah, why worry, that's one of main questions asked," van Vliet says. "You have to look at the bigger picture. And if you look on a global scale, 40 to 50 percent of all the plant and animal species are located on two percent of the earth's surface. And if 50 percent of all the plant and animal species are in danger because they are in a climate zone they're not used to, then I think we have a major issue there. Many species will be lost."

Van Vliet says the trick to driving home the importance of phenological data, and what it's telling us about the impacts of climate change, is to enlist the help of the public. To the end he has set up websites where citizens can help scientists track the tiniest changes in nature–things like the time and place of new hay fever symptoms or tick bites, or even the number of bugs smashed on a license plate after a summer drive.

They could all contain clues about how local environments are changing as the world warms up and the seasons continue to shift.

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